Avoid getting the AFSC 3P0X1 (Security Forces) at all costs. This means that don't go into basic as "open general", don't let your recruiter "unofficially" add it to your job preference list (this how they got me). This may mean waiting longer to leave for basic that's fine. Also, when you get to tech school push yourself as hard as you can to make sure you pass the course, because most people that fail out of their tech school training become 3P0X1. I have no doubts that my enlistment would be exponentially better if I got a job I wanted.
Also, don't sign up for 6 years. I know it should go without saying but they will attempt to entice you with signing bonuses, putting rank on faster (it's only 6 months faster) and it's not worth it. If you like the 4 years you spend enlisted you can reenlist. If you don't like your enlistment at your 4 year mark as a 6 year enlistee you still have another 2 years. It's much easier to add time in service than remove it. Sign up for 4 years.